1. Field of the Invention
The present invention relates to a laser apparatus and a production method of a laser apparatus.
2. Description of the Related Art
In general, a semiconductor laser device includes a p-type electrode, an n-type electrode, a p-type optical waveguide, an n-type optical waveguide, and an active layer.
Being provided near the active layer and made of materials different from the material constituting the active layer, the electrodes and optical waveguides act as a resonator for confining the light generated in the active layer.
By the above configuration, stimulated (or induced) emission of laser light is caused in the active layer, and the laser light is amplified in the resonator and then guided outside the device by the optical waveguides.
Moreover, there is also a configuration using a Fabry-Perot resonator in which a cleavage plane is used as a resonator.
However, since the above-mentioned resonators cannot spatially control light perfectly, further improvement is required in order to achieve such a laser device that is desired for use in optical communication and a light source for an optical disk, operates at a lower current, and has a higher output.
Therefore, recently, as an ideal laser device, attention has been focused on a photonic crystal which can control the propagation of an electromagnetic wave, in particular a light.
The photonic crystal is a structural member in which the refractive index of a constituent substance is distributed periodically and is an artificial material which can achieve novel functions only by means of structural design.
As a most remarkable feature of the photonic crystal, it has been known that a region referred to as a photonic bandgap, where a specific electromagnetic wave cannot propagate, is formed due to the difference in refractive index of the constituent material and the periodicity of the structure.
By introducing a suitable defect into a refractive index distribution in a photonic crystal, an energy level due to the defect (defect level) is formed in the photonic bandgap. This enables the photonic crystal to control an electromagnetic wave with perfect freedom. Besides, the use of the photonic crystal enables the size of a device to be reduced.
The largest advantage of using a photonic crystal as a resonator of a laser device is that since perfect spatial control of a light, which has been hitherto impossible, can be achieved, a laser light can be obtained which has an extremely small threshold value (theoretically zero) and small temperature dependencies of the output and the wavelength.
Moreover, the advantage is also in that since the radiation of light can be controlled with respect to the entire space, high electro-optical conversion efficiency can be obtained.
Although, one dimensional, two dimensional, and three dimensional photonic crystals are present, in order to achieve the above-mentioned effect to a maximum extent, the three dimensional photonic crystal is most suitable.
In other words, the three dimensional photonic crystal has a feature that the distribution of refractive index of the constituent substance has a three dimensional periodicity, and an electromagnetic wave existing at a position of a defect hardly leaks outside and is therefore most suitable for controlling the propagation of an electromagnetic wave.
A representative example of the three dimensional photonic crystal is a woodpile structure (or rod-pile structure) disclosed in U.S. Pat. No. 5,335,240.
The woodpile structure, as illustrated in FIG. 7, has a structure having a stacked stripe layers in which a plurality of rods are periodically disposed parallel to each other at a predetermined in-plane periodicity.
Specifically, the woodpile structure is configured such that each rod belonging to each stripe layer intersects perpendicularly to each rod belonging to a nearest neighbor stripe layer, and each rod belonging to each stripe layer is parallel to each other and is offset by a half of the in-plane periodicity with each rod belonging to stripe layers being apart therefrom by two layers.
The periodicity of the photonic crystal structure is an order of a half of the wavelength of an electromagnetic wave to be controlled. For example, the periodicity of a photonic crystal for visible light is about 250 nm.
In the prior art, as a semiconductor laser device using such a three dimensional photonic crystal, for example, in Japanese Patent Application Laid-Open No. 2001-257425, a semiconductor laser device having low threshold current characteristics, in which an active portion is provided in a three dimensional photonic crystal structure, and a production method thereof has been proposed. The proposed semiconductor laser device has a structure such as illustrated in FIG. 8, in which an undoped InGaAs etch stop layer 1102 is formed on the surface of a high resistance InP substrate 1101.
Moreover, on the surface of the etch stop layer 1102, a three dimensional photonic crystal structure containing an active layer 1304 is formed.
Furthermore, on the surface of the three dimensional photonic crystal structure, a p-type electrode 1105 and an n-type electrode 1106 are formed via an InGaAs contact layer.
In other words, here, on the photonic crystal structure, there are formed electrodes composed of thin films of materials different from the material of the photonic crystal structure.